In pistons of this kind, the piston rings have hitherto been made conical or spherical for obtaining a sealing action also when the piston is inclined with its longitudinal axis at an angle relative the centre axis of the cylinder. In the Swedish patent specifications Nos. 190,707 and 210,202, various designs of sealing piston ring assemblies having spherical peripheral surfaces for engagement with the cylinder wall are described. When applying the inventive concepts described in said pair of specifications in industrial mass production it has turned out, however, that the spherical piston rings in this connection exhibit some drawbacks.
Thus the spherical ring is relatively expensive to manufacture in mass production since it requires for its good operation a very careful machining of the spherical surface. Mainly said machining includes grinding which however is difficult to carry out since problems are encountered in securing the ring rigidly during grinding. Most particularly this applies to the smallest ring dimensions in the smallest axial piston machine sizes. Under all circumstances the grinding of the rings is a time-consuming and cumbersome procedure which hence is expensive.
In the known spherical piston ring there also occurs an unavoidable leakage through the gap of the radially slitted ring. The diameter of the ring is dimensioned such that the tangential extent or the width of said gap will be approximately zero when the longitudinal axis of the piston rod and the piston coincides with the centre axis of the cylinder. For increasing inclination angles therebetween, the ring gap will increase progressively due to the fact that over half of its periphery the ring must assume an elliptical shape in order to seal against the cylinder wall. The ring gap achieves its greatest width at the maximal inclination of the longitudinal axis of the piston and piston rod from the cylinder centre axis, this inclination being of the order of up to 4.degree..
The leakage losses through the gap are not alone of determining importance per se. At the high pressures used nowadays for the working fluid in for instance axial piston machines of the kind set forth, amounting to the order of 300-400 bar and in a not too remote future perhaps increased to 500-700 bar, such leakage through the gap means that erosion damages will occur due to the very high flow velocity of the leaking fluid. Such erosion damages primarily hit the cylinder walls but also the pistons get such damages. The aforementioned circumstance that the ring during its reciprocatory motion in the cylinder has to adopt over half of periphery an elliptic cross section at least two times per revolution of the cylinder drum also implies the occurence of bending stresses which in turn might cause fatigue ruptures of the ring.
Investigations also have proved that the spherical piston ring is worn due to the fact that the ring is in a state of pressure unbalance over half its periphery while in operation. Due to said wear the width of the gap will increase. A wear which moreover is rapidly self-increasing very soon leads to leakage losses through the gap of such a magnitude that they will have decisive influence on the operation of the machine.
In a purely parallel or translatory piston rod movement in the cylinder without the occurence of an inclination of the piston, there has been utilized in applications having relatively low pressures a piston ring assembly consisting of a plurality of mutually equal cylindrical rings such as disclosed in the German patent publication No. 2,440,037. Should attempts be made to apply said piston ring structure in spherical pistons, i.e. inclinable pistons, there has to be taken into consideration the same decisive drawbacks as to the width of the ring gaps and leakage caused thereby as well as the liability to erosions as in the spherical piston rings.